In a power supply device for driving an AC load, such as a motor or a power convertor, a half bridge circuit created by directly connecting first and second switching elements can be employed. This power convertor switches the input DC power via the first and second switching elements, and supplies AC power to the load, which is connected to a mid-point of the half bridge circuit. The first and second switching elements are, for example, IGBTS or MOS-FETS having high withstand voltages. The first and second switching elements are alternately turned ON/OFF by a drive control circuit which is implemented as a power IC, for example.
In this drive control circuit, an overcurrent detection circuit is normally employed to stop the first and second switching elements from turning ON when an overcurrent flowing through the switching elements is detected. This overcurrent detection circuit protects the load, connected to the power convertor, and also the switching elements when an overcurrent is detected. The overcurrent detection circuit is configured by a current detection resistor, which generates a voltage in proportion to the current flowing through the switching elements, and a comparator, which compares the voltage detected via the current detection resistor and a predetermined reference voltage to detect an overcurrent flowing through the switching elements. For example, if the switching elements are IGBTs, the current detection element is constituted by a current sensing IGBT.
In this case, it is necessary to construct the overcurrent detection circuit considering the output temperature characteristic of the current detection circuit, in order to improve overcurrent detection accuracy and prevent malfunction. Therefore it has been advocated that a standard voltage Vreg be applied to a resistance dividing circuit, which is constituted by resistors Ra and Rb connected in series, as shown in FIG. 7, and which generates a reference voltage by resistance-dividing the standard voltage Vreg via a bipolar transistor TR (e.g. see Patent Document 1, which is identified below). This overcurrent detection circuit is for providing a temperature characteristic, which corresponds to the output temperature characteristic of the current detection element, to the reference voltage.
In other words, the overcurrent detection circuit having the configuration shown in FIG. 7 provides a positive temperature characteristic to the reference voltage using the negative characteristic of the diode-connected bipolar transistor TR, whereby the positive output temperature characteristic of the current detection element is cancelled out. In FIG. 7, Rs denotes a current detection resistor which converts the current supplied from the current input terminal Tin into voltage. Comp denotes a comparator that compares the reference voltage generated by the resistance dividing circuit and the voltage in proportion to the current flowing through the switching element, which is detected via the current detection resistor Rs.
Patent Document 1: Japanese patent Application Laid-open No. 2004-45305